1. Field of the Invention
This invention relates to a charge transfer device, and more particularly to a charge transfer device including one-dimensional charge transfer elements for converting a color image into an electric signal.
2. Description of the Relates Art
A conventional charge transfer device which employs a one-dimensional charge transfer element having a plurality of pixels arranged in a row is described below. In order for the conventional charge transfer device to read a two-dimensional color image, reflection light from or transmission light through an original upon which white light is impinged is separated into three colors of red, blue and green, and the lights of the colors are converted into electric signals by the charge transfer elements and the electric signals are stored temporarily into a memory. As the one-dimensional charge transfer elements, or the original, are being moved, the reflection light or transmission light is then successively converted into electric signals, and the respective electric signals of red, blue and green obtained from the same places are composed by a computer to regenerate a two-dimensional color image. This method requires one one-dimensional charge transfer element for each of the three colors of red, blue and green. Further, the electric signals obtained by conversion of the lights of the three colors obtained from the same place must be composed into one signal. Therefore, as the distance (hereinafter referred to as line distance) between a pixel row of a certain one-dimensional charge transfer element and a pixel row of another one-dimensional charge transfer element increases, an increasing memory capacity is required to store color information. For example, if the line distance increases doubles, the necessary memory capacity also increases doubles. The increase in the memory capacity in turn may make of an image reading apparatus which employs the charge transfer device costly. Furthermore, if the original is fed slantwise, the larger the line distance becomes, the more resolution will deteriorate.
The line distance is thus important factor influencing picture quality and cost. Assuming that the length of a pixel in a longitudinal direction is equal to 1, the line distance is essentially an integer and will not be a decimal.
FIG. 1 is a plan view of a conventional charge transfer device which includes three one-dimensional charge transfer elements for reading a color image.
As shown in FIG. 1, the conventional charge transfer device includes one-dimensional charge transfer elements of first to third rows corresponding to the three primary colors of light, that is, red, blue and green respectively. It is to be noted that, the arrangements of the colors are different from product to product, and one-dimensional charge transfer elements of three rows are required to read a color original.
Each of the one-dimensional charge transfer elements includes a plurality of pixel elements 1 for converting optical signals into charges, an element separation area 2, CCDs (Charge Coupled Devices) 4, and a readout electrode 3 for controlling reading out of charges from pixel elements 1 into CCDs 4. Each of the CCDs 4 has transfer electrodes 191, 192, 111, 112 in this order. Readout electrode 3 is made of polysilicon in the same step as that for transfer electrodes 111, 112.
FIG. 2 shows a section of FIG. 1 taken along line G-Gxe2x80x2, i.e., a structure of a portion through which charge is read out from pixel element 1 to CCD 4 and a potential distribution during operation. FIG. 3 shows a section of FIG. 1 taken along line H-Hxe2x80x2, i.e., a cross section of CCD 4, and a potential distribution during operation.
In the conventional charge transfer device, each of the CCDs 4 has a two-phase driving structure. A CCD 4 consists of a N-type semiconductor substrate 5, P-type well 6, a photodiode N-type well 7 which forms pixel element 1, photodiode P-type region 8 which forms pixel element 1, transfer electrodes 191, 192, 111 and 112, N-type well 10, N-type diffusion layer region 12, and oxide film 13 formed by implanting ions of boron or the like in self alignment into transfer electrodes 191, 192.
Next, operation of the conventional charge transfer device will be described with reference to FIGS. 1 to 3.
Charges are generated in pixel elements 1 of FIG. 1 dependent on incident light amounts and a storage time, and are read out to CCDs 4 through the application of a voltage of 5 V to readout electrode 3. Signal charges 15 are read out into CCDs 4 under the control of readout electrode 3. The charges read out into CCDs 4 are transferred in one direction through the successive application of a voltage to transfer electrodes 191, 192 and transfer electrodes 111, 112 of CCDs 4, as shown in FIG. 3. The transferred charges are then converted into voltages by charge detectors (not shown) provided at the terminals of CCDs 4 to be successively read out. The charges generated in pixel elements 1 at the first to third rows are successively transferred by CCDs 4 disposed for the individual rows to be converted into voltages.
The conventional charge transfer device shown in FIG. 1, in order to read in a color image, an arrangement employs, wherein three one-dimensional charge transfer elements of the same structure are juxtaposed in three rows. With this arrangement, the line distance cannot be made equal to or smaller than double the length of pixel elements 1 in the longitudinal direction.
Another conventional example is shown in FIG. 4 wherein the line distance can be made equal to or smaller than double the length of pixel elements 1 in the longitudinal direction.
As shown in FIG. 4, in this charge transfer device, the one-dimensional charge transfer element of the first row is inverted in an upward and downward direction in the figures, to reduce the line distance between the first and second rows. This approach has a drawback in that, since the line distance between the first and second rows and the line distance between the second and third rows are not equal to each other, if an original is fed slantwise as mentioned above, the ratio in color displacement differs from color to color, resulting dirty image.
Yet another conventional example which intend to solve the aforementioned problem is shown in FIG. 5.
In the conventional charge transfer device shown, pixel elements 1 of the first, second and third rows are arranged side by side and all of the line distances between adjacent pixel elements 1 is made equal to the length of pixel elements 1 in the longitudinal direction. Charges generated in pixel elements 1 in the one-dimensional charge transfer element of the second row are read out into CCDs 4 through pixel elements 1 of the third row.
FIG. 6 shows a sectional view of FIG. 5 taken along line I-Ixe2x80x2 and a potential distribution. A readout electrode 3 is interposed between pixel elements 1 of the second and third rows to read out charges generated in pixel elements 1 of the second row into CCDs 4 through pixel elements 1 of the third row.
In this conventional charge transfer device, the line distance is equal to the length of pixel elements 1 in the longitudinal direction. Consequently, the required memory capacity is at the least and, even if an original is fed slantwise, a resulting image would not be dirty. However, if light to be incident on pixel elements 1 of the third row is not intercepted, when reading out charges from the one-dimensional charge transfer element of the second row, then a mixture of colors will occur in pixel elements 1 of the third row.
In short, the conventional charge transfer device is disadvantageous in that, since charges of the one-dimensional charge transfer element of the second row must be passed through the one-dimensional charge transfer element of the third row, a color mixture will occur, which results in defect in characteristic such as appearance of a mistaken color.
It is an object of the present invention to provide a charge transfer device wherein three rows of pixels can be arranged adjacently to each other without giving rise to a problem in characteristic such as color mixture or appearance of a mistaken color.
The charge transfer device according to the present invention comprises first to third pixel rows arranged adjacently to each other, a first charge coupled element for reading out and transferring signal charges generated in the first pixel row, and a second charge coupled element for reading out and transferring signal charges generated in the pixels of the second and third pixel rows. In addition, second readout electrodes for reading out signal charges generated in the second pixel row to the second charge transfer element are provided, each of which is placed between adjacent pixels of the third pixel row.
Accordingly, since the line distances between each of the adjacent first to third pixel rows can be made substantially equal to a pixel length without giving rise to problems in characteristics such as color mixture or appearance of a mistaken color, a required memory capacity can be reduced.
According to an embodiment of the present invention, the pitch between the electrodes of the second charge coupled element is equal to one half the pitch between the electrodes of the first charge coupled element, and further includes electrodes provided between the first readout electrodes and the third readout electrodes for temporarily storing signal charges therein.
Accordingly, signal charges generated in the second pixel train and signal charges generated in the third pixel train can be read out simultaneously.
According to another embodiment of the present invention, the readout channel of each second readout electrode is dimensioned such that the width thereof is greater at the portion adjacent to the second charge coupled element than at the portion adjacent to the second pixel row.
In this embodiment, since the potential in the proximity of the second charge coupled element is higher than that in the proximity of the pixel elements due to narrow channel effect, the readout time of signal charges from the pixels of the second pixel row to the second charge coupled element can be reduced.
According to another embodiment of the present invention, a potential difference is produced by ion implantation below a transfer gate to thereby improve the flowing rate of charge. Accordingly, possible deterioration of the residual image characteristic can be prevented which may be caused by a long time required to read out charge and a large amount of charge remaining in the channel portion (remaining image).
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.